This invention relates to tunnel boring and to compositions for use therein.
Shield tunnel boring machines are increasingly used in the boring of tunnels, because they offer many advantages such as the ability to bore in a wide variety of strata. A shield boring machine comprises a circular rotatable cutting head mounted on a cylindrical shield of similar diameter such that its axis of rotation coincides with the longitudinal axis of the shield. Within the shield there are contained means for feeding materials to the cutting head and means for conveying away the soil. Such machines perform well when the strata through which a tunnel is being bored is relatively hard and firm, but they do not perform so well in soft and crumbly strata which can make soil removal very difficult. One way of seeking to overcome this problem is to apply a consolidation agent to the soil via the cutting head. This stabilises the soil, allowing boring to be more easily effected and the soil to be more easily removed.
Known stabilising agents have included bentonite slurrys and polymer suspensions. These can lead to problems in certain soils, largely because they increase the soil water content appreciably. Some soils (such as clays) can become sticky and difficult to remove and clog up the cutting head, resulting in a substantial loss in efficiency. In a more recent development, polymer foams have been suggested. These have the advantage that considerably less fluid is placed in the soil per given volume. A typical foam formulation will comprise a foaming agent and a foam stabilising agent, the latter generally a surfactant. Thus, in operation, the foam is injected from ports in the cutting head into the face being bored.
It has now been found that a particular foaming material gives especially efficaceous results. The invention therefore provides a method of boring a tunnel through a stratum by means of a shield tunnelling apparatus, the process comprising the injection into the stratum at the cutting face of an aqueous material comprising
(a) from 0.005-0.05% by weight of a polyethylene oxide of weight-average molecular weight from 2,000,000 to 8,000,000; and
(b) from 0.05-0.5% by weight of a sulphate-containing anionic surfactant.
The invention additionally provides a foaming material, comprising
(a) from 0.005-0.05% by weight of a polyethylene oxide of weight-average molecular weight from 2,000,000 to 8,000,000; and
(b) from 0.05-0.5% by weight of a sulphate-containing anionic surfactant.
The invention additionally provides the use of a foaming material as hereinabove defined in the conveying of soil from the cutting face of a shield tunnelling apparatus.
Polyethylene oxides (PEO) are well-known items of commerce and a suitable material may be selected from the wide range available. Typical examples are the xe2x80x9cPolyoxxe2x80x9d (trade mark) materials of Union Carbide. The anionic surfactant may be selected from any such sulphate-containing surfactant known to the art. A particularly preferred type is polyoxyalkylene alkyl ether sulphate, where the polyalkyleneoxide chain has an average chain length of 2-3 alkylene oxide units. Typical commercial materials include the xe2x80x9cAlscopexe2x80x9d (trade mark) series of Toho Chemical Industry Co.
Preferably the quantities of PEO and surfactant present are from 0.02 to 0.04% and from 0.15 to 0.4% by weight, respectively. Most preferably, the PEO quantity is from 0.030-0.04%.
The rest of the aqueous material is generally water, but other art-recognised ingredients (such as biocides and complexing agents) can be added in art-recognised quantities. In a preferred embodiment of the invention, the aqueous material additionally comprises urea, to the extent of from 0.03-0.8%, preferably 0.06-0.25% by weight. The invention therefore also provides a tunnelling foaming additive consisting essentially of polyethylene oxide, sulphate-containing anionic surfactant and urea and water.
Another especially useful ingredient is a foam improver. Foam improvers are materials which are well known in the art for their ability to stabilise foams. They are typically amines with long fatty acid chains derived from natural fats and oils, for example coconut oil.
The aqueous material may be prepared and stored in advance as a single material, but it is preferred to provide it as two components which are mixed prior to use. In this case, one component comprises PEO and surfactant, and the other is a dilute aqueous solution of PEO, typically at a weight concentration of from 0.007-0.015%, preferably from 0.0150-0.06%. Both of course contain sufficient water to ensure that they are sufficiently fluid, and sufficient of the essential ingredients to give on mixing proportions of the size listed hereinabovexe2x80x94appropriate mixtures with respect to molecular weights and proportions are readily determined. The water content of the aqueous material should be such that it has a viscosity as measured by the Bohlin System 3 apparatus, using spindle 1 of from 600-1200 mPa.s., and in the case of the two component embodiment, the water contents of the individual components should be such as to achieve this. The two-component embodiment is exceptionally storage-stable.
In operation, the aqueous material is supplied to the machine, where it is diluted to typically a 3% solution of total aqueous material in water and foamed by conventional means and injected into the tunnel face via ports in the cutting head. In addition, foam can also be injected from ports in the shield to strengthen the tunnel walls.